Renaud Laplanche was the face of the online-lending industry, a telegenic French entrepreneur who argued the Internet would upend finance by allowing borrowers to connect directly with investors.

On Monday, Mr. Laplanche was pushed out as the CEO of the firm he founded, LendingClub Corp., after the board said it found problems with its lending practices and what it called the executive’s lack of disclosure surrounding a personal investment.

The ouster amplified concerns about the business model Mr. Laplanche pioneered, and the company’s shares tumbled 35%, lopping off nearly $950 million in market value.

LendingClub, by most measures the biggest and most successful in a wave of online lenders that have blossomed since the financial crisis, said a board review found that the San Francisco company sold an investor $22 million in loans whose characteristics violated the investor’s “express instructions.” The board found that some people at the company knew the loans didn’t meet the investor’s criteria and that the application date on $3 million of those loans had been altered to make them comply.

Mr. Laplanche discovered the initial falsifications and ordered the internal probe, but the board later determined that he didn’t give investigators a complete account of what he knew, according to a person familiar with the matter.