Was there class mobility in the middle ages?

Answer:

No. If born into nobility you stayed noble. If born as a peasant you stayed a peasant. The only event that allowed any upward movement was the plague. When so many people died that left spaces open for people who were clever and smart to move into that class.

More:

However, if one were to join the church, they could move throughout ranks in the church via. education (in church-y ways). Class mobility in the church was easier for those born into a high social class, however.

More:

I should mention as well that while Jacob (the first editor in the first paragraph) said that class movement started after the plague, in actuality it was caused by an influx of trade after the Crusades (because of luxury goods) and a resulting expansion of the middle class.

Answer:

Actually, there was social mobility, though it was limited. If you think about it, a man (or woman) being knighted, who was not already a holder of a noble title, was a commoner. Being knighted elevated the knight if not already from a noble family.

But there are other examples of social mobility. Owen Tudor, a Welsh commoner, got a job as household steward for Catherine of Valois, dowager queen of England. In time they were married. She had several children by him, all of whom were half siblings of King Henry VI of England. King Henry VI gave two of these children noble rank, making one, Edmund, Earl of Richmond. As the Wars of the Roses progressed, the time came that a son of Edmund, Henry, became the logical choice to have as king to end the wars, and he became King Henry VII, founder of the House of Tudor. So we had family progress from commoners to nobility and from nobility to kings in three generations.