He was married to Mary at Child Okeford in 1829.
I am slightly confused with ages and names.
There is a Thomas Mitchell who married a Martha Goodfellow by licence in 1829 in East Orchard (the licence may have more details). The witnesses are a Mary Mitchell and a M/R. G. Mitchell. I have attached the entry so that you can make up your own mind about the first letter, but there is certainly a Richard Green Mitchell, yeoman in the area of about the same age (b1795 Stour Provost s Richard George and Susannah Mitchell), so perhaps he is a brother.
The 1841 census shows Thomas and Mary living with Martha, 5; and the 1851 census shows Thomas aged 76, and Mary aged 72. This suggests they would have been in their fifties when they married in 1829. Are the couple who married in 1829 he same as the ones in the 1841/51 census? There is a Martha Mary Mitchell born to Thomas and Martha Mitchell, yeoman, in Wimborne in 1836; she must be a candidate.
Unless you have other information, the best fit would be that a Thomas and Mary had a son, Thomas, in about 1800-1805, who then married Martha in 1829. There is a candidate in next door Somerset, but I can't find one in Dorset.
Richard