There are many works of fiction which include "rags to riches" tales of members of the lower classes joining the ranks the nobility. Real-world historical precedents seem to be much more difficult to find.
I'm searching specifically for examples of ennoblement in High Middle Ages to Late Middle Ages Europe, from roughly the 10th century to the 15th century CE, wherein a member of the lower classes was elevated to noble rank. If the noble rank included a landed title, all the more impressive.
For my purposes, I am excluding advancing as a clergyman or purchasing letters of patent in later centuries, perhaps the two most common forms of social mobility up to a certain level. In particular, I am looking for instances where ennoblement came through outstanding military success.
From my preliminary research, this appears to have been much more likely earlier in the period before feudal structures and traditions became more firmly entrenched, and during times of great strife, such as the Hundred Years War, though even then I suspect that royal patronage would have been necessary. Though I had expected this form of social mobility to be rare, I've been surprised that I've been unable to find a single concrete example.
First of all, a mandatory word of caution on the notion of nobility. This pertains to France 1285-1322 (as discussed in the article I refer to below), but makes sense for other areas as well. Some landowners, especially from provincial elites, pretty much enjoyed unofficial recognition of their status by neighboring peers. Some royal officials weren’t nobles, but held positions, that required knighthood (knighthood, being predicated on the noble descent usually). Noble status could be granted by a local lord without confirmation by the king – in a messy medieval overlapping-jurisdictions way.
These caveats also suggest that search criteria might be relaxed/adjusted to get more meaningful results from available data.
The closest instance I know of is Bertrand du Guesclin – he was from the lesser nobility, but achieved truly impressive heights.
Bertrand du Guesclin (died 1380) was a low-born Breton knight, who initially earned his reputation as a mercenary commander. Charles V recruited him during the Hundred Years’ War and eventually promoted to the Constable of France in 1370. Du Guesclin was ransomed several times, ransom being a distinctive sign of a high status and powerful patronage. An outstanding military leader and Charles’ closest ally, he was buried alongside (!) the kings of France in the abbey of Saint-Denis.
The article Ennoblement by the Crown and Social Stratification in France 1285-1322: A Prosopographical Survey (2015) by Jan Rogozinski examines individual cases and the context of ennoblement through royal letters. For the sense of scale: Philip IV issued 12 letters of nobility, Louis X – 6, Philip V – 45.
Technically those letters were requested by the “candidates” themselves, but were strongly backed by powerful nobles or the king. Who initiated a transaction is not as important though, as the fact that usually it was mutually beneficial. Following are the motivations from both sides:
The overall dynamics, thus, is one of maintaining patronage/clientele networks. The other thing is how they were branded. Some recipients actually carried out their military duties and advanced their patrons’ cause. The text mentions Jean Berenger, Fabri and Arnaud du Portal, who were royal sergeants at arms; Guillaume Gobe, who defended royal authority, challenged by Robert of Artois; Guillaume du Vernet fought in the Flemish campaign under his lord (count of Auvergne). Closer investigation is warranted to differentiate a pretext from real military feat (whatever that might mean).
Another possibility of gaining land and certain vassalage status, attached to it, was available to early Spanish colonizers (which goes a bit beyond your specified period). Following is the quote from Henry Kamen’s Spain’s Road to Empire (2003):
These men, who proudly assumed the description of ‘conquistadors’, were often not even soldiers. The group of men that seized the Inca emperor at Cajamarca in 1532, was made up of artisans, notaries, traders, seamen, traders, gentry and peasants, a small cross-section of immigrants to America and in some measure a reflection of peninsular society itself. <...>
This meant that they were engaged on their expedition by virtue of the crown conceding them an encomienda, a contract that gave the recipient rights to demand tribute and labour from the natives, and obliged him to serve and defend the crown and instruct the natives in the Christian faith. The wording of the contract frequently specified a form of feudal service, ‘with arms and a horse’,1 making it evidently a military agreement.
European crusaders were strongly materially incentivized too, and the overall ad-hoc environment of the crusading enterprise promised additional social gains. I don’t know any specific examples, but would suggest looking there.