Contrary to what I was taught in middle-school US history (surprise surprise!), I've just learned that emancipation of slaves in northern states was a gradual, multi-stage process that could last into the 1840s.
I had always learned that part of the reason for the northern states moving away from slavery was because climate/agriculture was not suitable for the type of plantation system that developed in the south.
So, what were these northern slaves doing in the first half of the 19th century? Were they agricultural workers, or did they exist in skilled professions like barbering? Do we know if they predominately lived in urban or rural parts of the north?
What work did they do after their eventual emancipation? Did that vary by a person's age and employment prior to emancipation?
I know you're asking mostly about jobs, but I want to back up a little to explain how emancipation mostly went down in the North to help contextualize how we got here. The standard history book formula is something like this, right at the start of the Civil War chapter: "At independence, all the thirteen colonies practiced slavery. The states of the North ended it in the decades immediately after, making slavery a specifically Southern institution."
All of that is true, from a certain point of view. Looking backwards through the Thirteenth Amendment, it's easy to forget how wild it was. It freed every single slave in the country immediately and without compensation to their enslavers of any kind. Almost every previous emancipation involved neither of those. With the exception of Massachusetts and possibly New Hampshire -there's debate about just what did happen in NH; I can't tell you more because the only book I know of that goes into it is in the mail on its way to me just now- every state that abolished slavery did so by what's called gradual emancipation. Gradual emancipation had two major goals, besides freeing enslaved people and both were especially pressing to the people who voted them through.
So gradual emancipation laws, which start with Pennylvania's in 1780, manage those two concerns by setting a specific date where they take effect for people born that day or after. In PA's case, it's March 1, 1780. The slaves in Pennsylvania who had petitioned for relief, offered to pay special taxes for emancipation, and so forth are completely out of luck. They remain slaves for the rest of their lives. Their children born on or after the day aren't born as slaves, but rather indentured servants bound to their mother's owner until the age of 28. That's the owner's compensation in the form of their service for a large portion of their productive lifespan.
Under the auspices of the law, zero Pennsylvanian slaves (except for those who enslavers try to hide from the system registering them or otherwise cheat the law with; George Washington is the most famous of the latter) become free until March 1, 1808. Slavery of a kind would extend for decades beyond that, until the last fertile enslaved woman living in PA bore her last child. Thus you have things like the 1811 case of an ironmaster registering the birth of an enslaved girl so he can retain her until 1839. Enslavers are not shy about using these kids as something to hold over the parents, naturally. Pennsylvania came back in 1847 and took care of the few remaining slaves born too early to fall under the law and actually free the allegedly free people born into indentured servitude until they aged out.
Every state that did it had different particulars, but that's the basic model. As a result, it's hard to say when slavery actually ended in the North. It's technically correct to say that it happened in 1804 when New Jersey passed its gradual emancipation law but NJ still had 18 enslaved people in 1860, if you count those bound for life despite the abolition laws. Holdovers born too soon for gradual emancipation, those bound to labor for their parents' enslavers for decades, and shady "apprenticeship" systems that were slavery in all but name -Illinois had one of those- make the picture much murkier. The last of the subsequent laws to handle the leftover enslaved people and all-but-slavery systems pass in the late 1840s, so we can use the 1840 census as a fair indicator. It lists 1,166 enslaved people in the North, mostly in NJ and Illinois. In 1850, it's just 262, mostly in NJ. (The others are in Utah Territory.)
The reasons that the North moved away from slavery are many. The enslavers didn't like it one bit and fought hard, managing to delay New York's emancipation law until 1799, but overall the problem is that there are less of them than in the South. To a large degree it is down to a lack of plantation agriculture, as you note, though there are cases of large-scale agricultural operations with enslaved labor in the Mid-Atlantic prior to the Revolution. Back then, everyone thought that the region was going South, as it were, from the increasing prominence of enslaved labor. New England is different, though slavery rolled on there to a smaller degree. Aside from agriculture, Northerners stole enslaved labor for domestic services (to the point where enslaved domestics were the done thing even in Boston for a while) and in skilled trades. That includes, quoting from Leon Litwach's North of Slavery here,
bakers, carpenters, cabinetmakers, sawyers, blacksmiths, printers, tailors, and coopers - and perhaps most prominently in the maritime industry.
The expense and scarcity of white labor argued strongly for doing so. White workers naturally protested the competition. These are trades you can practice in small towns and the countryside, but the black population in much of the North is fairly urban. That's where the markets are bigger and where the well-heeled enslavers who bought them to work in their shops are more prone to living.
To some degree, there's prestige in many of these trades. It's not necessarily whole lot, but skilled trades have a cachet that unskilled labor does not. After emancipation, white competitors work to force the freedpeople out of them in order to have the market to themselves. As long as an enslaved mechanic was owned by a white man, patronizing him (skilled trades are usually male) wasn't really between you and the enslaved person. It was between you and the enslaver. The changed dynamic, plus growing hostility from white competitors now that they're out from under the sometime protection of their enslavers, makes most freedpeople downwardly mobile. They're forced out of the trades by informal prejudice and various institutional means, like all-white trade associations. People who had parents in skilled trades or themselves were mechanics end up reduced to domestic service, day labor, and so on.
Litwack again:
In New York, Philadelphia, and Boston, the men worked largely as laborers, mariners, servants, waiters, barbers, coachmen, bootblacks, porters, second-hand clothing dealers, and hot carriers, while the women worked as washerwomen, dressmakers, seamstresses, and cooks.
About 87% of the black population with jobs ended up with positions like that. This naturally creates a vicious circle where racism keeps black people out of the good jobs. Whites notice that there aren't any (or are few) black people so employed, and come to think that they just aren't up to the task. White supremacists cause the phenomena that they then cite as proof they were right about black people all along.
It's especially telling that this all takes place at the same time as the white working class develops its own identity and begins more aggressively asserting itself. In the eighteenth century, a servant could be white or black. Whites and blacks alike had masters, for whom they labored. After the Revolution, that shifts. Whites now have bosses. If they work in domestic trades, they are "help" and will angrily correct you if you call them a servant. The ultimate shakeout from this is that the generation or so after emancipation in the North are forced out of most positions that might offer them a chance at upward mobility. Their children and grandchildren up into the Civil War are still in the same spot.
Sources
Ira Berlin, Many Thousands Gone: The First Two Centuries of Slavery in North America
Leon Litwack's North of Slavery: Negro in the Free States 1790-1860 is still the standard text despite being vintage 1961.
David Roediger's The Wages of Whiteness: Race and the Making of the American Working Class