Slush Pool mined its first block in 2010 and has miner over 1.2 million BTC lifetime, the most of any mining pool.
The pool was founded by Marek "Slush" Platinus, who ran it until 2013. Then, Marek left the pool to found Satoshi Labs (the makers of Trezor hardware wallets), and operation of the pool was given to Braiins. Braiins is an embedded systems (e.g. ASICs) engineering company that also develops ASIC firmware in addition to running Slush Pool since 2013.