The entertainer tested positive for the illness last month and was said to have been “responding well to treatment”.
However, his death was confirmed on Friday and his representative said he died from “complications of Covid-19”.
Horn and Siegfried Fischbacher were world renowned for their Las Vegas shows involving white lions and tigers.
In a statement, Fischbacher said: “Today, the world has lost one of the greats of magic, but I have lost my best friend.
“From the moment we met, I knew Roy and I, together, would change the world. There could be no Siegfried without Roy, and no Roy without Siegfried.
“Roy was a fighter his whole life including during these final days. I give my heartfelt appreciation to the team of doctors, nurses and staff at Mountain View Hospital who worked heroically against this insidious virus that ultimately took Roy’s life.”