For the year to April 2011, the Welsh society lent £105m, up from £90m for the previous year.
It says this has led to a net increase in the value of its mortgage assets of £41m, taking its assets to £550m.
The society also reported a post-tax profit of £2.4m for the year to April 2011, up 26% on its £1.9m profit in 2010.
In addition, its results state there were six loans with repayments twelve months or more in arrears at the year-end, up from five loans in 2010, and there were five properties in possession at the end of the year, up from four in 2010.
At the year end there were 22 accounts in arrears where interest-only arrangements were in place and 104 accounts where other forbearance options had been implemented. The total amounts outstanding on all loans where forbearance had been exercised was £8.2m.
Andrew Lewis, chief executive at Monmouthshire Building Society, says: “These are excellent results in what remains a particularly challenging environment for all building societies where increased profitability and growth has been modest for many competitors.
“Monmouthshire Building Society continues to go from strength to strength, despite a difficult and challenging marketplace.”