Daniel Boulton was detained by Lincolnshire Police after his 26-year-old ex-partner Bethany Vincent and her son Darren Henson were stabbed to death in High Holme Road, Louth, at around 8.29pm on Monday.
The suspect was arrested in bare feet at around midday on Tuesday at a farm near Louth.
Lincolnshire Police said the suspect, who was known to both victims, was arrested in the Hubbard’s Hills area.
He was also held on suspicion of stabbing an off-duty police officer.
The force said both victims had suffered multiple stab wounds at an “absolutely devastating and dramatic scene”, and a knife believed to have been used to inflict the injuries was recovered.
An off-duty police officer was also taken to hospital after being stabbed in nearby Hubbard’s Hills.
Forensic officers also appeared at the scene of the arrest around four hours later.
An off-duty Lincolnshire Police officer was also injured in the Hubbard’s Hill area of Louth on Tuesday morning amid a manhunt for Boulton.
The force said he was being held in custody for questioning.