ParalympicsGB team-mate Claire Cashmore completed the podium in that race in a time of 1:07:36, while fellow Briton George Peasgood earlier won men’s silver in 58:55.
In addition Great Britain’s rowers grabbed double Paralympic gold as Fox grabbed glory in the PR3 mixed coxed four final alongside Ellen Buttrick, Giedre Rakauskaite, and Ollie Stanhope, plus cox Erin Kennedy.
They finished in 6:49.24, more than 11 seconds ahead of the United States and almost 18 seconds clear of bronze medallists France.
Laurence Whiteley made it a 30th birthday to remember by topping the PR2 mixed double sculls podium alongside Lauren Rowles, recording a winning time of 8:38.99.
Whiteley and Rowles also won mixed sculls gold together at Rio 2016 and once again blew the competition out of the water on Sunday.
They crossed the line just under five seconds ahead of Dutch duo Annika Van Der Meer and Corne De Koning, with Chinese pair Liu Shuang and Jiang Jijian almost 10.5 seconds off the pace in the bronze medal position.
And elsewhere wheelchair star Hannah Cockroft picked up the sixth gold medal of her illustrious Paralympic career by powering to T34 100 victory in a world-record time of 16.39.
The 29-year-old lowered her own global best by 0.18 seconds at the National Stadium in Tokyo, with compatriot Kare Adenegan in second.