Bobby Clark was apparently on the verge of making the Scotland No.1 spot his very own for the foreseeable future, when out of the blue he suffered a dramatic loss of form and McGarr was promoted to the first team. There can be little doubt that Clark's confidence problem had its roots in an infamous clash with Celtic's Bobby Lennox at Parkhead on September 28, 1968, when the impish forward nipped in and punched the ball from the Aberdeen keeper's grasp to score a controversial, late winning goal. But Clark's nightmare was McGarr's good fortune and in early December 1968 Ernie was handed the first team jersey.

He quickly established himself as a superb custodian in his own right and it became difficult to see how the replacement could be asked to step aside even if a spell in the reserves restored Bobby Clark's confidence. Ernie's style was somewhat different to his predecessor with greater flamboyance and emphasis on the spectacular reaction save coupled with a bigger physical presence. His displays for the Dons won him deserved attention, and his meteoric rise was complete when he was called up for the full Scottish side to face the Republic of Ireland in Dublin on September 21, 1969.

Unfortunately, the occasion was more of a trial than a triumph for McGarr. Although the game was only a friendly international, it was seen by manager Bobby Brown as key preparation for a vital World Cup qualifier against West Germany the following month. Incredible as it may seem, the Irish game was played in Dublin on a Sunday after all the players had taken part in their usual club fixtures the day before! Ernie had the bad luck to pick up a leg injury in the Don's battle with St Johnstone at Muirton Park on the Saturday, and took the field with his right thigh heavily strapped. It quickly became obvious that manger Brown's gamble on his new keeper's fitness was going to fail, and after a mere 25 minutes between the sticks during, which McGarr received another knock on his injured leg, the Don's custodian was replaced by rival Jim Herriot.

McGarr could not be risked for the World Cup qualifier in Hamburg, and the Scots failed to qualify for Mexico, 1970 when they lost a dramatic game 3-2. On November 5, 1969 Ernie was put in goal for the now meaningless final qualifying tie against Austria in Vienna, but he failed to distinguish himself in a poor team performance and before another opportunity arose his moment had gone.

The early weeks of 1970 saw the big keeper suffer a loss of form himself, and Bobby Clark re-emerged from the wilderness to which McGarr returned. Later the same year, with Clark back to his best, Ernie saw little prospect of another miracle and not surprisingly tabled a transfer request. In January, 1971, his wish was granted with a move to Dunfermline Athletic, but he spent less than 18 months at East End Park before making a more permanent move to East Fife. Ernie made the short trip to Cowdenbeath in May, 1977 and was actually between the sticks at Pittodrie when old mate Joe Harper scored the goal against the Fifers that gave him the Dons scoring record. McGarr signed for Airdrie in September 1978, before making his final move to Berwick Rangers two years later. He retired at the end of the 1980-81 season.