Newton was a member of the British elite, he had income from Cambridge University, and he was first the Warden and later the Master of the Royal Mint. He was also experienced in investments.

Nevertheless, he also got sucked into the infamous South Sea mania of 1720, which was basically the monkey JPG of its day, and consequently he lost a great deal of his wealth.

“Newton's financial misadventures in the South Sea Bubble”: https://royalsocietypublishing.org/doi/10.1098/rsnr.2018.001...