He first appeared in the penultimate heat game of Series 76, winning two games and amassing 243 points — giving him the highest average score of any contestant in the series — before stepping aside for the series finals. He maxed 14 rounds in his second game. He then returned for the opening heat of Series 77 — the first game to be played on the "new" blue set — on 3 July, showing no sign of slowing down as he spotted the nine-letter word MACROLIDE. In the sixth game of the series, Chafer-Cook rounded off his run with a total of 896 points, becoming the first octochamp and 800 club member of Series 77.
As a result of his success in the preliminaries, Chafer-Cook returned for the Series 77 finals in December as #1 seed. In the quarter-final stage, he maintained his edge, delivering a 105 – 60 defeat to fellow unbeaten player Graeme Staples with the help of such spots as POTLINES and TRAINEAU. Chafer-Cook's next match pitted him against #4-ranked Noel McIlvenny; here, he asserted his dominance even further, prevailing early on with the nine UNDERSEAL and racking up 134 points (only one point short of the game's maximum score) to beat his own record for the highest score of the series. This brought Chafer-Cook to the series grand final, where he was pitted against the series' #2 prospect, Bradley Horrocks. Despite falling five points behind in the first round due to his disallowed offering of resexed ☓, he proceeded to score in every subsequent letters and numbers round, ultimately eking out a nine-point lead by beating Horrocks with POOLIES and ENLIVEN. The conundrum was to remain a crucial one. However, when neither contestant was able to unscramble RILEYISMS in the time, the final score was confirmed at 100 – 91, and Chafer-Cook consequently obtained the Richard Whiteley Memorial Trophy and the title of Series 77 champion. His “xicount” total is 1,235 points.
Chafer-Cook returned to Countdown in January 2019 for Championship of Champions XV, taking on fellow Apterite and series winner Martin Hurst in the championship's sixth preliminary. Both players put up an impressive fight, unravelling the niners ROYALISED and ELASIPODS, but Chafer-Cook sealed victory prior to the final numbers game and unscrambled ITIESCORE to finish with 129 points and advance to the quarter-finals. He played against Matthew Tassier in the second quarter-final. He went ahead very early on, despite both players messing up in the first number rounds. Chafer-Cook picked up speed in the second half when he risked the nine letter word OPERATEES. Eventually he racked up 111 points to Tassier's 62 points, maxing all 10 letters rounds, and set up a rematch of the Series 77 grand final against Bradley Horrocks. This game was on a knife’s edge: Chafer-Cook won the opening round with UNASKED, but blanked on the Horrocks’s 6-small pick in the third numbers round. Then, in the final letters round, Horrocks unravelled ATHEROMA to extend his lead to 11 points. Both players maxed the last numbers round, before Chafer-Cook bowed out with one final conundrum solve to finally lose by a single point.
|#||Date||Type||Contestant 1||Score||Contestant 2||Guest||Lex||Max|
|6606||19/06/2017||P||John Trueman||38 – 117||Tom Chafer-Cook||Suzi Perry||Susie Dent||125|
|6607||20/06/2017||P||Tom Chafer-Cook||126 – 52||Bobbi Ramsammy||Suzi Perry||Susie Dent||134|
|6615||3/07/2017||P||Tom Chafer-Cook||118 – 37||Carmel O'Brien||Greg Rusedski||Susie Dent||134|
|6616||4/07/2017||P||Tom Chafer-Cook||129 – 21||Nick Knight||Greg Rusedski||Susie Dent||134|
|6617||5/07/2017||P||Tom Chafer-Cook||110 – 55||Rob Spereall||Greg Rusedski||Susie Dent||132|
|6618||6/07/2017||P||Tom Chafer-Cook||107 – 51||Al Harris||Greg Rusedski||Susie Dent||132|
|6619||7/07/2017||P||Tom Chafer-Cook||91 – 73||Martin Smith||Greg Rusedski||Susie Dent||123|
|6620||10/07/2017||P||Tom Chafer-Cook||98 – 17||Barry Taylor||Suzannah Lipscomb||Susie Dent||130|
|6730||14/12/2017||QF||Tom Chafer-Cook||105 – 60||Graeme Staples||Paul Zenon||Susie Dent||121|
|6734||20/12/2017||SF||Tom Chafer-Cook||134 – 83||Noel McIlvenny||Rufus Hound||Susie Dent||135|
|6736||22/12/2017||GF||Tom Chafer-Cook||100 – 91||Bradley Horrocks||Rufus Hound||Susie Dent||122|
|6985||9/01/2019||CP||Tom Chafer-Cook||129 – 89||Martin Hurst||Dr Linda Papadopoulos||Susie Dent||142|
|6990||16/01/2019||CQF||Matthew Tassier||62 – 111||Tom Chafer-Cook||Jon Culshaw||Susie Dent||128|
|6993||21/01/2019||CSF||Bradley Horrocks||111 – 110||Tom Chafer-Cook||Jon Culshaw||Susie Dent||128|