Signifying another major change at HBO, longtime pay cabler exec Colin Callender is departing after a 21-year run.

The HBO Films president, who announced his departure Tuesday, said he plans to launch his own entertainment and content shingle next year.

Exec won’t be replaced; instead, his lieutenants will be bumped up to lead their respective departments: Kary Antholis will become president, HBO Miniseries, and Len Amato will become president, HBO Films.

Both Antholis and Amato will now report to programming group/West Coast operations prexy Michael Lombardo.

News of Callender’s ankling isn’t a complete surprise; his potential departure had been rumored since last year’s executive shuffle at the channel, when he was among the contenders to lead HBO in the wake of Chris Albrecht’s hasty exit. Channel instead went with a team led by CEO Bill Nelson, as well as Richard Plepler and Lombardo.

Since then, it’s been understood that Callender has been itching to explore other opportunities.

Move also comes several months after Picturehouse — the theatrical label run by HBO and New Line — was shut down. But Callender said HBO Films’ exit from the feature business didn’t play a role in his departure.

“I want to return to my entrepreneurial roots and capitalize on my experience as a producer,” Callender said. “The truth is, there’s been one constant that’s shaped my career, and that’s change. And the industry is at another key inflection point, the marriage of traditional entertainment and new digital content.”

The exec said he’ll reveal details of his new venture next year but said there will be a strong digital component. Callender will stick around HBO for a bit longer to help with the executive transition, he said; beyond that, there will be no formal ties.

According to Callender, he hasn’t signed a production deal or first-look arrangement with HBO. But both sides said they’re open to working together on new projects.

“I think it’s obvious we’re disappointed; Colin has been a huge part of this company for a long time,” said HBO co-president Plepler. “He made an enormous contribution here. It’s our hope and expectation (that) there will be many joint collaborations with Colin’s new company.”

Callender’s departure reps the latest shuffle at the channel, which had remained relatively stable in the exec department until Albrecht’s exit. Earlier this year, longtime HBO Entertainment topper Carolyn Strauss transitioned to a production gig; former UTA partner Sue Naegle was tapped to fill that job.

But unlike the channel’s series stable — which had hit a bumpy road following the retirement of staples such as “The Sopranos” and “Sex and the City” — HBO’s longform output has remained consistent.

Callender’s coming off another banner Emmy season, as HBO scored a record-setting 13 wins for the longform “John Adams,” including miniseries, as well as three for “Recount,” including made-for-TV movie.

Callender first joined HBO in 1987 as exec producer of “HBO Showcase.” He was later named head of HBO NYC Prods. As HBO Films topper since 1999, he’s overseen development and production for films and miniseries for the network (and until recently, theatrical films as well).

Under Callender’s watch, HBO longforms have scored 84 Emmy Awards and 26 Golden Globes. His output has included “Miss Evers’ Boys,” “Conspiracy,” “The Gathering Storm,” “Elephant,” “Angels in America” and “Maria Full of Grace.”

Prior to HBO, Callender was an independent producer in the U.K., winning his first Emmy in 1983 for the nine-hour TV adaptation of the Royal Shakespeare Co.’s “The Life and Adventures of Nicholas Nickleby.”

As for Amato, exec had been senior veep of HBO Films since March 2007, serving as an exec producer on “Recount.” He joined the pay cabler from Spring Creek Prods., where he was president. Antholis joined HBO in November 2004 and has served as senior VP of HBO films and miniseries, handling projects such as “John Adams” and “The Pacific.”

“I leave the division in very good hands,” Callender said. “We’ve got ‘The Pacific’ coming up and a strong slate of movies in the coming year.”

Plepler and Lombardo reaffirmed their commitment to the longform genre despite Callender’s departure and the elimination of Picturehouse.

“Clearly, there was a time we were producing for Picturehouse as well as the service, so our volume went up,” Lombardo said. “But we have a full slate for next year, including our most expensive mini (‘The Pacific’) ever in post. We’re looking for the next major miniseries as well. Don’t read into this that there’s any change in our investment in longforms. It’s a vital part of HBO.”