Although the Union began as a Democratic paper, by the early 1910s it had been purchased by Londonderry politician Rosecrans Pillsbury, a Republican.

In October 1912, the competing Manchester Leader was founded by Frank Knox, later Secretary of the Navy during World War II, and financed by then-Governor Robert P. Bass, a member of the Progressive (or Bull Moose) Party who was attempting to promote the Progressive cause in New Hampshire.

Following Knox's death in 1944, William Loeb purchased the company, merging the Union and Leader into a single morning paper, the Manchester Union-Leader, in 1948.

In January 2025, majority ownership of the paper moved from the Nackey S. Loeb School of Communications, a nonprofit organization, to two private investors.

[3] Two notable early employees of the New Hampshire Sunday News were Ralph M. Blagden, the first managing editor,[4] and an even more prominent journalist he mentored, Benjamin C. Bradlee.

The paper's hard-hitting editorials, sometimes written by the publisher and featured on the front page, drew national attention and frequently prompted harsh criticism: The Manchester Union Leader, practitioner of a style of knife-and-kill journalism that went out of fashion half a century ago in the rest of the country, is the primary daily paper of 40 percent of New Hampshire's population...After 2018, when the newspaper laid off its full-time editorial writer, the Union Leader's brash editorial tone softened.

In a message printed in the paper in early 2009, publisher Joseph McQuaid announced that owing to financial difficulties affecting the entire newspaper industry, the Saturday edition of the paper would no longer be distributed outside of the Greater Manchester area and that Saturday content would be moved to a combined Friday/Saturday edition.

In 2017, the Union Leader building was sold to investor Peter Levine for $3.8 million after being on the market for about four years.

[13] After repeated rounds of layoffs over several years, the newspaper moved into smaller quarters in Manchester's downtown Millyard in the summer of 2024.