Tom's jobs change as he passes from owner to owner. On the Shelby farm, Mr. Shelby describes Tom to the slave trader Mr. Haley as his farm manager, saying that he
manages my whole farm like a clock.
As Tom and his family are having dinner in their cabin, the narrator describes Tom as "Mr. Shelby’s best hand."
From what Mr. Shelby and others say about Tom, he carries a great deal of responsibility for the running of the farm. Mr. Shelby trusts him to carry large sums of cash and allows him to come and go from the farm without worrying that he will run away.
When Tom is bought by Augustine St. Clare, the first idea St. Clare has is to put him in charge of the horses and make him his coachman. He says to Tom,
I think I shall put you in coachy.
However, Augustine has bought Tom because his beloved little daughter, Eva, has taken a liking to him. Tom saves Eva when she falls off the side of a boat, which increases St. Clare's desire to have him. As coachman, this gives Tom more time for the work that is most important to the St. Clares: they want Tom to be at Eva's beck and call. Tom likes this job because he likes Eva, and she is considered to be the soul of goodness and purity.
Simon Legree buys Tom with the idea of making him an overseer but, upon observing Tom's moral character, puts him to hard labor in the cotton fields instead.