Dewji was born on 8 May 1975 in Ipembe, Singida. He is the second of six children of Gulamabbas Dewji and Zubeda Dewji. Dewji received his primary education in Arusha at the Arusha Primary School and continued his secondary education at the International School of Tanganyika (IST) in Dar es Salam, Tanzania.

In 1992 his Father enrolled him at the Arnold Palmer Golf Academy in Orlando, Florida, where Dewji also attended Trinity Preparatory School for 11th Grade. Dewji then moved for his last and final year of high school to the Saddle Brooke High School in Florida.

Dewji attended Georgetown University in Washington D.C, graduating in 1998 with a bachelor's degree in international Business and Finance and a minor in theology.

Dewji is responsible for increasing MeTL's revenues from $30 million to over $1.5 billion between 1999 and 2018. MeTL Group has Investments in Manufacturing, agriculture, trading, Finance, mobile telephony, insurance, real estate, transport and Logistics, and food and beverages. The group is conducting Business in 11 countries and employs over 28,000 people with the aim to target over 100,000 people by 2021. MeTL's operations contribute ~3.5% of Tanzania’s GDP.

In 2000, Tanzania hosted its second multiparty elections where Dewji, at the age of 25 competed to become the Member of Parliament (MP) for Singida Urban. Despite winning the preliminary votes for the ruling party, Chama Cha Mapinduzi (CCM) with overwhelming majority, Dewji was deemed too young to hold the parliamentary seat.

In 2001, Dewji married Saira, his high school sweetheart with whom he has three children together and reside in Dar es Salaam, Tanzania. Dewji is a Shia Muslim from the Khoja Shia Ithna-Asheri Jamat as known as the 'Twelvers' sect.

Tanzania held its third multi-party elections in October 2005 and Dewji stood for the parliamentary seat again and was chosen by CCM to stand as a candidate for Singida Urban. In the general election he won with 90% of the votes and was sworn in as an MP for Singida Urban constituency on 29 December 2005. Dewji served for ten years before resigning from politics in October 2015.

Dewji established the Mo Dewji Foundation in 2014, with the goal of alleviating Tanzanian citizens from poverty and hardship. The focus of his charitable activities covers education, health and community development projects across Tanzania. Dewji has spent more than US$ 3 million on projects that have directly improved the lives of Tanzanians.

According to Forbes, Dewji has an estimated net worth of US$ 1.54 billion (2018), and is the 17th richest person in Africa and Africa's youngest Billionaire (2018). He was the first Tanzanian on the cover of Forbes Magazine, in 2013 and has been featured on three separate occasions. November 2015, Dewji was recognized as the Forbes Africa person of the year In his acceptance speech, he dedicated the award to the youth of Tanzania.