Over the years, Simon has appeared as a musical guest on The Tonight Show with Jay Leno, Late Night with David Letterman, Jimmy Kimmel Live, and Late Night with Jimmy Fallon, and has been featured in Rolling Stone, Paper, Mojo, Uncut, Clash, The Independent, and London Times.

He contributed music to two films by Abel Ferrara, New Rose Hotel and The Blackout, and played guitar on Carl Perkins's final album Go Cat Go!,[10][11] recorded at Sun Studios in Memphis, Tennessee.

Other musicians on the album include Marc Ribot, Steve Nieve from Elvis Costello's Attractions, Yuka Honda from Cibo Matto, and Petra Haden.

The debut featured songs co-written with Inara George and Booker Prize-winning novelist and poet Ben Okri.

It features drummer Pete Thomas from Elvis Costello's Attractions, Nikolai Fraiture from The Strokes, Bright Eyes' Nate Walcott, and Wilco's Mikael Jorgensen, among others.

[20] In 2012, Simon reunited with the Plastic Ono Band, and was a featured guest at three Yoko Ono shows—at The Brooklyn Academy of Music, L.A.'s Orpheum Theater, and San Francisco's Fox Theater—where he shared the bill with Iggy Pop, RZA, Thurston Moore, Kim Gordon, Eric Clapton and Perry Farrell, among others.

In 2012, Simon performed at the Jerry Garcia 70th birthday tribute along with Bob Weir, Cass McCombs, and members of Vampire Weekend.

Simon has presented several films in Los Angeles in the past several years, and has conducted Q&A's and has served as a panel moderator for many of the screenings.

He was the panel moderator at the screenings of Jimi Hendrix and Monterey Pop presented by Cinespia at the Hollywood Forever Cemetery in 2015.

In 2017, Harper played guitar on Lady Gaga’s title track for her album Joanne, produced by Mark Ronson.

Some of his interview subjects include former CIA director James Woolsey, former senator Al Franken, actresses Natasha Lyonne and Anjelica Huston, directors Sean Baker and Sam Taylor-Johnson, artists Ed Moses and Alex Israel, Devo’s Mark Mothersbaugh, Pink Floyd’s David Gilmour, Monty Python's Eric Idle, and many others.

He has interviewed artist Tracey Emin for Purple Magazine, and virtual reality pioneer and author Jaron Lanier for the L.A. Review of Books.