Johnson had signed a legally binding agreement to buy the stake on June 23 subject to approval. British media had reported the deal to be worth close to 190 million pounds ($256.37 million), with Johnson purchasing a 43% stake in Palace.
Johnson, former US Ambassador to the UK, joins chairman Steve Parish, Josh Harris and David Blitzer as a partner and director of Palace.
"It is an organisation with a proud history, tradition, and deep roots in English football in South London, which I came to admire during my time as US Ambassador to the United Kingdom," Johnson said in a statement.
"...I look forward to working with him (Parish) and the entire ownership group to build on the club's recent successes and help shape an exciting future for Crystal Palace."
