He became a journalist in his native Dundee but later decided to try his hand at the same profession in the United States, to where he emigrated at the age of 21. He did not find work as a journalist and instead found employment in the motor car industry in Detroit. He then attended the Detroit Technical College, to study engineering.
Although he was now beginning to be established in the United States he took a keen interest in the developing political movement for Scottish independence back home. To that end he joined the newly formed National Party of Scotland in 1928, as an overseas member, and Scotland's political and economic plight was never far from his thoughts.
In 1932, Donaldson married Vi Bruce, another expartiate Scot (from Forfar)and set up home in Washington D.C, where he worked for the Chrysler Corporation. In the mid-1930s, now with a family Donaldson returned to his native Scotland, moving to Ayrshire to work in farming.
Donaldson came into contact with Robert McIntyre, one of the leading members of the SNP and his involvement with the party deepended.
During the war, Donaldson's home was raided by police who suspected him (And a number of other SNP figures) of subversive activities. He was arrested and was conveyed at first to Kilmarnock Prison and thence to Barlinnie Prison in Glasgow where he was held for six weeks.
Donaldson's arrest and detainment did not disuade him of the value of his political activities. He remained a member of the SNP throughout the 1940s and 1950s when they were particularly weak and much of the focus of nationalist efforts were being invested in the Scottish Covenant of John MacCormick (who had left the SNP in 1942 as he had been unable to persuade the party to adopt a position of supporting devolution rather than independence, a split which Donaldson himself had put down more to personality clash than any of ideology).
After the split of 1942 Donaldson became a leading SNP figure along with McIntyre (who became SNP leader in 1948, serving until 1956). Donaldson became SNP leader in 1960, replacing James Halliday, and it was during his term as SNP leader that the party began to grow and impose itself on the Scottish political landscape.
During his term of office the SNP began to perform credibly in elections, winning the 1967 Hamilton by-election, and polling more votes than any other party in the 1968 Scottish local authority elections.
This success did not leave Donaldson without his critics though, and at the 1967 SNP Annual Confenrence he faced a leadership challenge from Douglas Drysdale which he comfortably defeated. In 1969 though he was replaced as SNP leader by Billy Wolfe.