With a series of lengthy fog delays throughout the day limiting playing time at Crans-sur-Sierre Golf Club on Thursday, only eight groups completed 18 holes before play was suspended due to darkness just after 8pm local time.
Half the field did not start their rounds at all on day one and will get under way on Friday morning.
Mansell carded six birdies and one bogey while Spaniard Cantero made an eagle, four birdies and a bogey to join the Englishman on five under par.
Home favourite Ronan Kleu and South African Richard Sterne were in a tie for third one shot further back after 13 holes, alongside Italian Guido Migliozzi, who signed for a 66.
Two-time former champion Matt Fitzpatrick, who has three holes to complete on Friday, was among those on three under.
Mansell started from the tenth tee and made an opening bogey before bouncing back with birdies on the 12th, 15th and 17th to turn in 33.
He then finished with three birdies in his final four holes to get to five under.
Cantero, meanwhile, produced fireworks on the back nine as he holed a long-range birdie putt at the 13th before making an eagle on the 14th.
Alex Noren held off Nicolai Højgaard and a charging Kazuma Kobori to clinch his second British Masters title - and first DP World Tour title in seven years - with a one-shot victory at The Belfry.
Marcel Siem holds a one-shot advantage heading into the weekend at the Betfred British Masters, after carding a 69 to follow his opening 66 on Friday.
Haotong Li is in a four-way tie for the lead alongside Matthias Schwab, Thomas Aiken and Marcel Siem after the first round of the Betfred British Masters on Thursday.
Marco Penge took advantage of Rasmus Højgaard's dramatic collapse to secure his second DP World Tour title at the Danish Golf Championship.
