Her appointment follows the resignation last week of Tracey Crouch, who left the job saying she was unhappy with the government delaying measures to reduce maximum stakes on fixed-odds betting terminals.
Davies was previously an assistant government whip, and in July became parliamentary under secretary of state for Wales. She will take on the full remit of Crouch, which in addition to youth work policy, includes civil society and sport. It also includes the National Lottery and gambling. The role will remain within the Department for Digital, Culture, and Media.
Youth leaders had hoped the government would use Crouch's departure as an opportunity to create a dedicated youth minister, amid concerns that reform of the youth sector could slip down the government's agenda with the minister having such a large portfolio.
James Cathcart, director of Young Voices Heard, said he is "disappointed that the Prime Minister did not take the opportunity of Tracey Crouch's resignation to reflect on whether the remit of Sport, Civil Society and in particular, youth, would be better served with separate Ministries".
He added: "I hope that this issue stays on the agenda, and in the meantime, might herald more visible pragmatic and resourced partnerships with the sector, to share the load, and in particular giving a higher profile to young representatives voice in government."
- Civil Society Strategy sets out plans to remake youth work landscape
- Is new youth legislation needed?
The British Youth Council (BYC) has said it is to write to Mims outlining its long-standing concerns with the size of the brief Crouch inherited.
"It's a real shame that the government have decided to ignore calls for a youth minister," said Lewis Addlington-Lee, BYC deputy chair.
"In order for young people's voices to be championed effectively across government, it is important this brief is made smaller. We can't see how this can be carried out when the minister will continue to have so many responsibilities"
BYC chair Amanda Chetwynd-Cowiesonsaid she hopes that Mims ensures young people's views are heard across government.
"The British Youth Council is looking forward to working with the new minister to ensure the government remains committed to young people's voices being hard right across the government," she said.
"This is a crucial time for the country, it's vital young people are part of the decision making that is taking place at all levels of the government."
Crouch took over the youth policy brief in June last year from Rob Wilson, who lost his seat in the 2017 general election.
Others to hold the youth policy brief since 2010 include Brooks Newmark, Tim Loughton, Edward Timpson and Nick Hurd.