He came to prominence for writing, directing, producing, editing, and scoring no-budget short horror films, which he released online under the pseudonym "ponysmasher".

[5] Starting in 2006, Sandberg found moderate online success making animated short films; according to Sandberg, his first online successes were his films För Barnen and Vad Tyst Det Blev, the latter of which garnered 1.7 million views on YouTube.

[7] His last collaboration with Rosberg before his retirement was Animera = Göra Livlig, a 2010 documentary about senior citizens creating animated films.

Lacking any funding of his own, his shorts were made with no budget: his equipment and props consisted mostly of items he already had or could buy very cheaply, and his only cast and crew were himself and Lotta Losten, his wife.

According to Sandberg, his life changed "in every way imaginable" after Lights Out; he began receiving tons of messages from film industry people who wanted to work with him.

[19] In April 2019, TheWrap reported that Sandberg would direct a sequel for New Line Cinema called Shazam!

Fury of the Gods, Sandberg stated he was "done" with the superhero genre and expressed a desire to return to horror.

[22] Sandberg is set to direct and produce the horror film The Culling with his wife Lotta Losten through their production company Mångata.

[23] He is set to direct the film adaptation of Cullen Bunn and Jack T. Cole's graphic novel The Unsound[24] and thriller Below,[25] both for Netflix.

[26] Sandberg met actress Lotta Losten in the early 1990s, when they were both around 11 years old, and they later began a relationship before marrying in 2013.