sinclair was born in the london borough of lambeth and grew up in clapham which is based in the borough. he attended pimlico school in westminster and was offered trials at chelsea, arsenal and wimbledon whilst he represented the west london district team. he later joined chelsea as an eleven-year-old, stating that supporting the club was a major influence in his decision making. he progressed through the youth age groups going on to sign as an apprentice in april 1988 before turning professional in may 1990. in april 1991, after a run of four consecutive defeats, chelsea manager bobby campbell decided to blood some youngsters for the end of season clash with luton town. sinclair was chosen out of position at left-back, but impressed on debut as chelsea fought back from 3–0 down to draw the match 3–3 despite having ten men for the whole second-half. he kept his place in the side for the next three fixtures at left-back but was subbed off at half-time in a 7–0 away defeat to nottingham forest and then rested for the remainder of the season.