Maclagan was one of the longest-serving international rugby players during the early development of the sport, and was awarded 25 caps for Scotland.

He played international rugby for thirteen seasons, a Scottish record for sixty years,[2] and led the first official British Isles team on its 1891 tour of South Africa.

[8] Maclagan continued at three-quarters, and after facing England and Ireland in both the 1880–81 and 1881–82 seasons he was selected for the first Scottish international with Wales as part of the inaugural Home Nations Championship in 1883.

The Welsh team were beaten by three goals to nil, with Maclagan converting all three tries to earn himself his first international scores.

Maclagan was on the score sheet again the next match when he converted one of the tries in a victory over Ireland, but was part of a losing Scottish team when they were beaten by England in the Triple Crown and Championship decider.

Maclagan was given the captaincy of Scotland for the 1884 Home Nations Championship, which saw identical results to the previous season; a win over Wales and Ireland, but a loss to eventual tournament winners England.

The team drew against England, but the result was enough to give Scotland the Championship for the second time, making Maclagan a Home Nations champion.

1887 saw the official formation of the International Rugby Board, an event which had its origins in the 1884 argument between Scotland and England.

His final match at England gave Maclaran his twenty-fifth cap,[10] a record for a Scottish player at the time.

In the final test of the series against South Africa at Cape Town he scored one of two tries, the other coming from Randolph Aston.