Moniak, from La Costa Canyon High School in south Carlsbad, became the first prep outfielder chosen No. 1 overall since Tampa Bay drafted Delmon Young in 2003.
The selection, announced by Commissioner Rob Manfred at MLB Network studios, marks the first time the Phillies have led off the draft since they took Miami slugger Pat Burrell in 1998.
With no consensus No. 1 talent this year, there was plenty of uncertainty about who the Phillies would grab right up until they officially went on the clock.
"I definitely wouldn't say there's pressure," Moniak said in an interview on MLB Network. "I'm excited to hopefully prove the Phillies right."
The left-handed-hitting Moniak is 6-foot-2 and 190 pounds and is considered a five-tool talent whose stock rose throughout his senior season. He consistently hits to all fields with solid power to the gaps. Moniak is also an outstanding defensive player who has good speed and is expected to remain in center field as a pro.