Porter, who began his playing career in Tottenham's youth setup before spells at AFC Wimbledon, Barnet and Bromley among others, has become assistant manager to Mark Robins at Stoke City after leaving Arsenal in June.
Horseman steps up from his role as an elite player development coach to replace him. He arrived at the club in 2025, having previously managed Southampton's B side for four years before taking his first senior job at Forest Green Rovers in July 2023. That spell proved short-lived, lasting only five months before he was sacked in December of that year having won just two of his opening seven matches in charge.
Horseman subsequently worked as a first-team coach at Bristol Rovers and briefly became the club's interim head coach in 2024 alongside former Sunderland manager Lee Cattermole, following Matt Taylor's dismissal.
He left after the club were relegated to League Two, before moving back into youth football and eventually joining Arsenal's academy staff.
Speaking on his promotion, Horseman said he felt honoured to take on the role.
"I'm absolutely delighted and privileged to be appointed Arsenal Under-21s head coach, following in the footsteps of so many outstanding coaches who have held this position before me," he said.
"It's a huge honour to contribute to the continued success of an academy with such a proud history of developing players for our first team and the wider game."
Horseman becomes the latest in a long line of Arsenal Under-21 head coaches since Arsène Wenger's departure in 2018, a period that has seen far more turnover in the role than during the previous three decades.
He inherits a side that finished 14th in Premier League 2 last season, with new academy director Pascal De Maesschalck hoping the appointment will help raise standards as Arsenal continue to prioritise developing players for Mikel Arteta's first team.