Australian midfielder Tim Cahill's first-half goal propelled Millwall into their first-ever FA Cup final as the Lions beat First Division rivals Sunderland 1-0 at Old Trafford.

Millwall now face Premiership giants Manchester United in next month's final at Cardiff's Millennium Stadium.

Cahill, who only made his Australia debut in the midweek friendly against South Africa, fired home in the 26th minute as they beat Sunderland, managed by former Millwall boss Mick McCarthy, for the third time this season.

Victory also guaranteed the Londoners a UEFA Cup place next season with United already assured of European qualification.

And it meant player-manager Dennis Wise, three-times an FA Cup winner, had another chance at lifting the trophy.

"I can't put into words what this means," Wise told Sky Sports afterwards.

Sydney-born Cahill, who on Tuesday ended his nine-year battle for international recognition with Australia by coming on as a substitute in the 1-0 friendly win over South Africa at London's Loftus Road, played down his crucial goal.

"It's all about being in the right place at the right time," said Cahill, 24, who followed up Paul Ifill's strike to lash the ball home.

"It's unbelievable. You've got to take things like this in your stride. Manchester United are my heroes," added Cahill, who ensured Millwall's second semi-final since their last appearance at this stage in 67 years also against Sunderland, ended in victory.

Sunderland were forced to play the closing stages with 10 men after experienced Republic of Ireland midfielder Jason McAteer was sent off for a second bookable offence.

Brought to you by AAP © 2024 AAP