Nicholas Hawkes, 39, of Basildon, Essex, sent unsolicited photos of his erect penis to a 15-year-old girl and a woman on Friday.
He is the first person to be convicted of the new offence of cyber-flashing, which was brought in under the Online Safety Act and came into effect on 31 January.
After pleading guilty to two counts of sending a photograph or film of genitals to cause alarm, distress, or humiliation, he was remanded in custody until 11 March, when he will be sentenced at Basildon Crown Court.
Sefer Mani, of the Crown Prosecution Service (CPS) in the East of England, said: "Cyber-flashing is a grotesque crime and the fact we were able to deliver swift justice for the two victims shows the new law is working.
"I urge anyone who feels they have been a victim of cyber-flashing to report it to the police and know that they will be taken seriously and have their identities protected."
Hawkes is a registered sex offender until November 2033 after he was convicted and given a community order for sexual activity with a child under 16 and exposure last year at Basildon Crown Court, the CPS said.