The mayor was taking a trip on the SL10, which links Harrow and North Finchley. It launched just over a week ago and was the first genuinely new section to be added to the limited-stop orbital “loop”.
It will be followed on Saturday (December 9) by the SL1, which will link North Finchley and Walthamstow Central.
The first Superloop route, SL8, which was a rebranding of the 607 that linked Uxbridge and White City, launched in July.
It was followed soon after by the SL6, a rebranding of the peak-hours X68 express bus that links West Croydon and Russell Square.
In August came the rebranding of two more existing routes: the X26, which links West Croydon and Heathrow, became the SL7, while the X140, which links Harrow with the airport, became the SL9.
TfL revealed on Tuesday that there had been a 62 per cent increase in weekly passengers on the SL7 since it became part of the Superloop network, which saw it double in frequency to four buses an hour in each direction.
Most other Superloop routes have outperformed the six per cent year-on-year passenger growth across the capital’s 700 routes.
The SL6 and SL8 are up 15 per cent, while the SL9 is up three per cent.
Eventually there will be 10 Superloop routes – seven orbital sections and three “spokes” linking the suburbs with central London.
Nine sections will be running by next April, including the SL2 between Walthamstow Central and North Woolwich, the SL3 between Thamesmead and Bromley and the SL5 between Bromley and Croydon.
The last section – the repurposed X239, linking Grove Park and Canary Wharf – will start running in 2025 as the SL4 when the Silvertown tunnel opens.
Mr Khan said: “It is fantastic to see the Superloop already encouraging more people onto London’s bus network. It’s proving a real game-changer.
“Now people travelling in outer London are getting another boost before Christmas with two brand new routes coming into service. I’m delighted to see so many Londoners benefitting from the Superloop so far, and there’s much more to come.”
Fares on Superloop buses cost the same as a conventional London bus - £1.75. The buses have been rebranded in retro livery and some of the routes have brand-new electric double deckers.
The Superloop is part of a £6m investment in outer London bus travel. Critics say it will only boost capacity by less than one per cent, in terms of its city-wide impact.
There were 415m London bus journeys between July and September, up six per cent on the 391m in the same three months a year earlier.
Louise Cheeseman, TfL’s director of buses, said: “We will continue our work to bring the rest of the Superloop routes into service as quickly as possible."