Simmonds had a promising junior career, spending time as the number ranked doubles player on the ITF rankings. He was a member of the United States team that finished runners-up in the 2002 Junior Davis Cup and in the same year won the doubles at the Orange Bowl, with Scott Oudsema.[2] Simmonds and Oudsema would go on to claim the 2003 Australian Open boys' doubles title, the first American male pairing to win the competition.[3] They defeated the Romanian pairing of Florin Mergea and Horia Tecău in the final.[2] In 2003, he also reached the boys' doubles semi-finals at the French Open and Wimbledon, partnering Brian Baker.[2]
Simmonds was given a wildcard into the 2006 US Open main draw, but couldn't get past 25th seed Richard Gasquet in the opening round.[4] He competed in the men's doubles at the US Open three times, in 2004, 2006, and 2007, but fell in the first round each time.[4]
He has won three Challenger titles in his career.[4] In 2006 he won the singles title at the León Challenger tournament and he has also won two doubles titles, at Nouméa in 2007 and Baton Rouge the following year.[4] As of the end of 2012, he has won nine ITF Futures titles, three in singles and six in doubles.[4]