When the conservatives returned to office in 1949 under Robert Menzies after eight years in opposition, McEwen became Minister for Commerce and Agriculture, switching to Minister for Trade in 1956.
Menzies nicknamed him "Black Jack", due to his dark eyebrows, grim nature, and occasional temper.
In the Menzies Government, McEwen pursued what became known as "McEwenism" – a policy of high tariff protection for the manufacturing industry, so that industry would not challenge the continuing high tariffs on imported raw materials, which benefitted farmers but pushed up industry's costs.
This policy was a part (some argue the foundation) of what became known as the "Australian settlement" which promoted high wages, industrial development, government intervention in industry (Australian governments traditionally owned banks and insurance companies and the railways and through policies designed to assist particular industries) and decentralisation.

In 1958, following Fadden's retirement, McEwen was elected unopposed as leader of the Country Party.
This made him the de facto deputy prime minister, and gave him a free choice of portfolio.
Fadden had been Treasurer, but McEwen somewhat unexpectedly chose to continue on as trade minister.
This allowed Harold Holt to become the first Liberal MP to serve as Treasurer; since then every Treasurer in a Coalition Government has been a Liberal.
McEwen nonetheless had considerable influence in cabinet.
He and his party favoured interventionist economic policies and were opposed to foreign ownership of industrial assets, which placed him frequently at odds with his Liberal colleagues.
In 1962, a dispute between McEwen and Assistant Treasurer Les Bury ended with Bury being sacked from cabinet.
His stature eventually grew to the point where he was considered a potential successor to Menzies as prime minister.
An opinion poll in December 1963 showed that 19 percent of Coalition voters favoured McEwen as Menzies' successor, only two points behind the poll leader Holt.
By December 1965, this number had risen to 27 percent, compared with Holt's 22 percent.
McEwen's cause was championed by a number of media outlets, including The Sun and The Australian.
Nonetheless, he had few supporters within the Liberal Party, and it was generally held that he would have to become a Liberal if he were to lead the Coalition, which he was unwilling to do.

Holt replaced Menzies as prime minister in January 1966, with McEwen continuing on his previous position.
His portfolio had been expanded after the 1963 election, with his department now called the Department of Trade and Industry.
McEwen enjoyed a "sound working relationship" with Holt, but without the same rapport he had had with Menzies.
However, he had a poor relationship with William McMahon, Holt's replacement as Treasurer.
They had philosophical differences over free trade and foreign investment, both of which McEwen opposed.
McMahon was also suspected to be undermining McEwen through his connections in the media.

McEwen's most serious disagreement with Holt came in November 1967, when it was announced that Australia – which had converted to decimal currency the previous year – would not follow the recent devaluation of the pound sterling.
This effectively marked Australia's withdrawal from the sterling area.
McEwen issued a public statement criticising the decision, which he feared would damage primary industry.
Holt considered this a breach of cabinet solidarity, and made preparations for the Liberal Party to govern in its own right in case the Country Party withdrew from the government.
The situation was eventually resolved In Holt's favour.
When the conservatives returned to office in 1949 under Robert Menzies after eight years in opposition, McEwen became Minister for Commerce and Agriculture, switching to Minister for Trade in 1956. Menzies nicknamed him "Black Jack", due to his dark eyebrows, grim nature, and occasional temper. In the Menzies Government, McEwen pursued what became known as "McEwenism" – a policy of high tariff protection for the manufacturing industry, so that industry would not challenge the continuing high tariffs on imported raw materials, which benefitted farmers but pushed up industry's costs. This policy was a part (some argue the foundation) of what became known as the "Australian settlement" which promoted high wages, industrial development, government intervention in industry (Australian governments traditionally owned banks and insurance companies and the railways and through policies designed to assist particular industries) and decentralisation.
In 1958, following Fadden's retirement, McEwen was elected unopposed as leader of the Country Party. This made him the de facto deputy prime minister, and gave him a free choice of portfolio. Fadden had been Treasurer, but McEwen somewhat unexpectedly chose to continue on as trade minister. This allowed Harold Holt to become the first Liberal MP to serve as Treasurer; since then every Treasurer in a Coalition Government has been a Liberal. McEwen nonetheless had considerable influence in cabinet. He and his party favoured interventionist economic policies and were opposed to foreign ownership of industrial assets, which placed him frequently at odds with his Liberal colleagues. In 1962, a dispute between McEwen and Assistant Treasurer Les Bury ended with Bury being sacked from cabinet. His stature eventually grew to the point where he was considered a potential successor to Menzies as prime minister. An opinion poll in December 1963 showed that 19 percent of Coalition voters favoured McEwen as Menzies' successor, only two points behind the poll leader Holt. By December 1965, this number had risen to 27 percent, compared with Holt's 22 percent. McEwen's cause was championed by a number of media outlets, including The Sun and The Australian. Nonetheless, he had few supporters within the Liberal Party, and it was generally held that he would have to become a Liberal if he were to lead the Coalition, which he was unwilling to do.
Holt replaced Menzies as prime minister in January 1966, with McEwen continuing on his previous position. His portfolio had been expanded after the 1963 election, with his department now called the Department of Trade and Industry. McEwen enjoyed a "sound working relationship" with Holt, but without the same rapport he had had with Menzies. However, he had a poor relationship with William McMahon, Holt's replacement as Treasurer. They had philosophical differences over free trade and foreign investment, both of which McEwen opposed. McMahon was also suspected to be undermining McEwen through his connections in the media. 
McEwen's most serious disagreement with Holt came in November 1967, when it was announced that Australia – which had converted to decimal currency the previous year – would not follow the recent devaluation of the pound sterling. This effectively marked Australia's withdrawal from the sterling area. McEwen issued a public statement criticising the decision, which he feared would damage primary industry. Holt considered this a breach of cabinet solidarity, and made preparations for the Liberal Party to govern in its own right in case the Country Party withdrew from the government. The situation was eventually resolved In Holt's favour.
John McEwen