He became a KC in 1914 and found the circumstances of World War I led him to extensive practice in prize law.
The war also generated many complex contractual disputes and MacKinnon developed a reputation for handling such cases with skill.
[1] He began to establish a reputation as a jurist and to advise the government on mercantile law, especially its international dimension.
[1] In October 1924, the minority Labour government was suffering the repercussions of the Campbell case and was not expected to survive.
Criminal law and juries had never formed a material part of his practice but he adapted well though his reputation as a judge never matched his standing as a lawyer.
In a notorious libel trial in 1943, the court was viewing a photograph from the magazine Lilliput showing a well-known male fashion designer juxtaposed next to a pansy.
When the Temple Church was bombed during The Blitz, he welcomed it with mixed feelings:[1] Smirke, Cottingham, Willement, and the rest of the gang … To have got rid of their awful stained glass windows, their ghastly pulpit, their hideous encaustic tiles, their abominable pews and seats (on which alone they spent over £10,000) will be almost a blessing in disguise.
[1] "In appearance MacKinnon possessed bushy eyebrows, penetrating eyes, a pronounced angular nose, and firm mouth.