The first functional sewing machine was invented by Elias Howe in 1846. It did drastically change the production of clothing, though not necessarily how you might expect. Prior to this point everything, of course, was hand-sewn. This doesn't mean it took a long time, an 18th century plain gown takes about ten hours and a full three piece suit about 40 hours, but it does mean that all fitted clothing was custom fit to a person. It would be a large enough waste of time to make something like a suit and wait for someone to come along that not only fit it well enough but liked the style and fabric. One size fits all/most items like kerchiefs and aprons could be ready made and some cities, like London, had a second-hand clothing market. But, the vast majority of clothing was made of fabric the customer purchased and took to a tailor or mantua maker to have sewn up to their specifications.

Once the sewing machine was invented, it was not immediately adopted. Just like we see computers as a potential threat to taking over jobs, it was a concern that many skilled workers would be put out of a job or receive lower wages. Sarah Hale wrote in 1867 that a hand-sewn shirt would take 10-14 hours. One by machine would take an hour. Seems great as a consumer who is now paying much cheaper prices. However, for women being payed by the shirt they have to quickly adapt to and purchase the machine, and even then Hale notes that their pay was still low. They are a much less skilled worker in the eyes of industry, simply operating a machine, despite the fact that it is an entirely new skill to be learned. Here's an article about the adoption of sewing machines.

What the machine allows for is mass-produced clothing. Which means that the clothing can now be made up without a specific customer in mind. So, we see a stock of off the rack clothing sitting in stores instead or advertised in catalogues. This really takes off in the 20th century when clothing is no longer fitted to the body, but is either loose or stretches to the wearers shape. For thousands of years prior clothing was custom fit by a professional and within a century of the invention of the sewing machine it was mostly pre made in general sizes or had to be made at home.

The way clothing is constructed also changed drastically. Different finishing, methods of fastening, and interior construction. I can honestly say that it takes me ten hours to make an 18th century gown by hand and it takes me six hours to make a modern style dress on a machine. Funny thing is it takes me ten hours to make an 18th century gown by machine. I've done dozens both ways and am very fast at both, but the way it's designed isn't condusive to machine work.