He first appeared on Countdown on 29 July 1996, defeating Peter Cottee 49 – 36 in the 21st heat of Series 32. He won three more games in impressive fashion before losing 45 – 28 to Craig Douglas. Sinclair's four wins were enough to see him return for the finals as No. 3 seed, where he beat David Ligat in the quarter-finals and 'took revenge' on Craig Douglas in the semi-finals, beating him 55 – 39. In the final, Sinclair took on Dennis Fearon, and after a close-run affair, it came to a crucial conundrum, with Fearon leading 73 – 68. It all looked over for Sinclair as LOSTAGAIN was uncovered and Fearon buzzed in on 3.5 seconds, but his answer of NOSTALGIC was incorrect, the clock was restarted and almost immediately Sinclair buzzed in to give the correct answer of NOSTALGIA, snatching the win and becoming the 32nd champion of Countdown in the process.
Sinclair returned the very next week for Championship of Champions VIII, but was defeated in the first round by eventual winner Chris Rogers.
He is also a competitive Scrabble player and appeared on the first series of Only Connect in 2008, as the captain of the "Edinburgh Scrabblers" team. His other members were ex-Countdowners Simon Gillam and Melanie Beaumont. They faced fellow Countdowners Elisabeth Jardine, Noel Turner and Penny Downer in their first game, and David Stainer in their second game.
|#||Date||Type||Contestant 1||Score||Contestant 2||Presenters||Guest||Lex||Max|
|1803||29/07/1996||P||Alan Sinclair||49 - 36||Peter Cottee||Richard Whiteley||Carol Vorderman||June Whitfield||Kendall Clarke||72|
|1804||30/07/1996||P||Alan Sinclair||68 - 17||Eileen Williams||Richard Whiteley||Carol Vorderman||June Whitfield||Kendall Clarke||73|
|1805||31/07/1996||P||Alan Sinclair||54 - 47||Stephen Cook||Richard Whiteley||Carol Vorderman||June Whitfield||Kendall Clarke||74|
|1806||1/08/1996||P||Alan Sinclair||65 - 57||Daniel Hodgin||Richard Whiteley||Carol Vorderman||June Whitfield||Kendall Clarke||85|
|1807||2/08/1996||P||Craig Douglas||45 - 28||Alan Sinclair||Richard Whiteley||Carol Vorderman||Philip Franks||Susie Dent||85|
|1843||23/09/1996||QF||David Ligat||37 - 45||Alan Sinclair||Richard Whiteley||Carol Vorderman||Philip Franks||Susie Dent||81|
|1846||26/09/1996||SF||Alan Sinclair||55 - 39||Craig Douglas||Richard Whiteley||Carol Vorderman||Philip Franks||Susie Dent||70|
|1847||27/09/1996||GF||Alan Sinclair||78 - 73||Dennis Fearon||Richard Whiteley||Carol Vorderman||Philip Franks||Susie Dent||117|
|1848||30/09/1996||CQF||Alan Sinclair||32 - 60||Chris Rogers||Richard Whiteley||Carol Vorderman||Jan Harvey||Susie Dent||83|