Net lending of £6.5bn in the past year was more than double the levels of the previous year.
Nationwide also increased its lending to first-time buyers by 75 per cent, with mortgages granted to more than 42,000 prospective homeowners.
Chris Rhodes, executive director of Nationwide, said the figures showed the mutual had a “major role” to play in the UK housing market.
He added: “We have not simply focused on new customers but, as you would expect from a member-owned organisation, we have reserved the very best deals for our existing customers.
“We have also given additional help to first-time buyers through our Save to Buy scheme and we have now extended that scheme to home-movers.”
Derek Gair, partner of Hampshire-based GDC Associates, said: “I don’t think it’s particularly healthy to have Nationwide dominating 15.1 per cent of the mortgage market, and another 5 lenders holding the remainder of it. It doesn’t foster competition or innovation. We need more diversity.”