Slotheby is first mentioned in December 1374, when he conveyed a messuage in Grimsby to a local mason and his wife. He was already by then trading as a skinner and is again described as such in a deed of May 1394 whereby he and his wife acquired premises at the east end of Grimsby market-place. Little is otherwise known about him before he became bailiff of Grimsby in 1380. Two years later he served as a juror at an inquisition ad quod damnum, held to determine if certain land in the borough could be granted to the church of St. James. All in all, he is known to have performed jury service on at least four other occasions, the last being in about 1402 when he was required to assess the value of property for taxation purposes.
Slotheby’s position was such that he often found himself in demand as a mainpernor, notably for other MPs returned for Grimsby. He stood surety for Robert Burton on his election in both 1393 and 1397 (Jan.), and for William Elmsall when he sat in 1395. Moreover, in April 1399 he acted as a guarantor for Thomas Fyssher and John Kelby, who had been accused (but not found guilty) of attempting to murder Geoffrey Askeby during the previous summer while he was in office as mayor.