Antwi was born in Kumasi, Ghana. When he was 14 years old, he moved to Spain and lived there for three years. He initially played street soccer, before he entered the youth setup of Spanish club Real Zaragoza for 1 year. He eventually obtained a Spanish passport.
At 17 years of age, he was spotted by Paco Herrera, then head scout of Liverpool. He watched Liverpool beat Chelsea home at Anfield in the semi-final of the 2005 UEFA Champions League. The experience of hearing the Anfield crowd left a great impression on him, and he expressed a desire to reach the status of former African Liverpool player, Guinean international striker Titi Camara. He joined Liverpool, who had newly been crowned the UEFA Champions League winners, when Liverpool manager Rafael Benítez offered him a contract in August 2005.
He started his Liverpool career, playing in the reserve team during the 2005-06 season. Due to a striker shortage, he initially played up front. In his usual defending position, he helped Liverpool win the 2006 FA Youth Cup, beating Manchester City in the final.
On 8 November 2006, Antwi was called up by the Under-19 Youth setup of the Spanish national team to train with them. The call-up was possible due to his Spanish passport. Despite the Spanish Under-19 call-up, Antwi can freely choose which country to represent at a senior level, before he turns 21 years old.
On 10 January 2007, Antwi was named captain of the Liverpool Reserves, for the first time and the 18-year-old led by example at the back against the experienced Neil Shipperley of Sheffield United. On March 13, it was announced that he had joined Accrington Stanley on a month's loan. He remained at Stanley until the end of the season.
In July 2008 he joined Tranmere Rovers on loan until January 2009.