Newburn started the final day at 1am on Wednesday (California time) and ran until the evening to knock off the final 120 kilometres.
As he started the final 5km, a large group gathered to chant him to the finish line.
Newburn started the 4500km journey on September 1. It had been a long held dream for the man who only took up running in his late 40s. He turned 60 during the run.
He originally hoped to break the world record at just over 46 days but by about halfway had found himself too far behind. The last few weeks had been geared to breaking the masters record which stood at 52 days 11 hours and 58 minutes.
Averaging 90km a day, the tough-as-teak Kiwi battled heat, wind and exhaustion as he crossed 12 states - at no point did it look as though he wouldn't make it.
Speaking to the Chronicle from Los Angeles, Newburn said the final 5km was a long 5km but it was a great end to the journey.
"It was great to finish - pretty emotional at times. Running up those stairs, it was the easiest stairs I've ever run."
He now has two days recovery in Los Angeles and some time to see the sights before the long flight home. He is expected back in Feilding on Monday.
"The body doesn't feel too bad - that's relatively speaking, of course."
The run also raised thousands of dollars for Koru Care and Sunshine Heroes.
