What is the lowest thrust for a first stage rocket for a rocket that reached Earth orbit?
The U.S. Vanguard rocket reached orbit three times with a first stage thrust of only 125 kN.
The first stage of the three-stage Vanguard Test vehicle was powered by a GE X-405 28,000 pound (~125,000 N) thrust liquid rocket engine.
@2012campion's answer shows that this was not the lowest thrust
The smallest rocket to reach orbit is the Japanese SS520-5. It had a peak thrust of $185 kN$ according to the same web page:
Firing up its first stage, SS-520-5 shot up from its launch rail at 2:03:00 p.m. local time on Saturday with its aft fins sending the climbing rocket into a spin to provide stabilization as it climbed with a thrust outweighing the vehicle’s mass by a factor of seven. The first stage, standing 6.1 meters tall and holding 1,587 Kilograms of propellant, pushed the vehicle skyward with a peak thrust of 185 Kilonewtons (18,900 Kilogram-force), averaging at 143kN (14,600kgf) over the course of a 31.7-second burn.
I can't rule out that a slightly larger rocket nevertheless had a slightly lower thrust, but this must be close to the lowest.
Note: When this answer was written, the question read as follows:
What is the lowest first stage rocket engine thrust for a rocket that reached Earth orbit?
The first stage vernier engines on early Atlas boosters were 526 lbf (2.3 kN).