He became a Death Eater on accepting the task.
The Pottermore writing on Draco Malfoy clarifies that Draco only became a full member of the Death Eaters when the Dark Lord asked him to kill Dumbledore and he accepted the task. Much like Regulus Black before him, Draco willingly joined the Death Eaters, but began to have misgivings after already having joined.
Voldemort, seeking to punish Lucius Malfoy still further for the botched capture of Harry, demanded that Draco perform a task so difficult that he would almost certainly fail – and pay with his life. Draco was to murder Albus Dumbledore – how, Voldemort did not trouble to say. Draco was to be left to his own initiative and Narcissa guessed, correctly, that her son was being set up to fail by a wizard who was devoid of pity and could not tolerate failure.
Furious at the world that seemed suddenly to have turned on his father, Draco accepted full membership of the Death Eaters and agreed to perform the murder Voldemort ordered. At this early stage, full of the desire for revenge and to return his father to Voldemort’s favour, Draco barely comprehended what he was being asked to do. All he knew was that Dumbledore represented everything his imprisoned father disliked; Draco managed, quite easily, to convince himself that he, too, thought the world would be a better place without the Hogwarts Headmaster, around whom opposition to Voldemort had always rallied.
In thrall to the idea of himself as a real Death Eater, Draco set off for Hogwarts with a burning sense of purpose. Gradually, however, as he found that his task was much more difficult than he had anticipated, and after he had come close to accidentally killing two other people instead of Dumbledore, Draco’s nerve began to fail. With the threat of harm to his family and himself hanging over him, he began to crumble under the pressure. The ideas that Draco had about himself, and his place in the world, were disintegrating. All his life, he had idolised a father who advocated violence and was not afraid to use it himself, and now that his son discovered in himself a distaste for murder, he felt it to be a shameful failing. Even so, he could not free himself from his conditioning: he repeatedly refused the assistance of Severus Snape, because he was afraid that Snape would attempt to steal his ‘glory’.
- Draco Malfoy (wizardingworld.com)
This confirms that Draco only became a Death Eater when he accepted the task of killing Dumbledore.
Before that, he’d supported the Dark Lord.
Though he only became a Death Eater after agreeing to kill Dumbledore, Draco had supported the Dark Lord’s views himself for much longer than that, though during that time the Death Eaters viewed him only as a schoolboy.
His own father had felt his Dark Mark burn and had flown to rejoin the Dark Lord, witnessing Harry and Voldemort’s graveyard duel.
The discussions of these events at Malfoy Manor gave rise to conflicting sensations in Draco Malfoy. On the one hand, he was thrilled by the secret knowledge that Voldemort had returned, and that what his father had always described as the family’s glory days were back once more. On the other, the whispered discussions about the way that Harry had, again, evaded the Dark Lord’s attempts to kill him, caused Draco further twinges of anger and envy. Much as the Death Eaters disliked Harry as an obstacle and as a symbol, he was discussed seriously as an adversary, whereas Draco was still relegated to the status of schoolboy by Death Eaters who met at his parents’ house. Though they were on opposing sides of the gathering battle, Draco felt envious of Harry’s status. He cheered himself up by imagining Voldemort’s triumph, seeing his family honoured under a new regime, and he himself feted at Hogwarts as the important and impressive son of Voldemort’s second-in-command.
- Draco Malfoy (wizardingworld.com)
In fact, Draco had been raised to support the Dark Lord and been in the company of others who did, since his childhood, before he started attending Hogwarts.
Draco was raised in an atmosphere of regret that the Dark Lord had not succeeded in taking command of the wizarding community, although he was prudently reminded that such sentiments ought not to be expressed outside the small circle of the family and their close friends ‘or Daddy might get into trouble’. In childhood, Draco associated mainly with the pure-blood children of his father’s ex-Death Eater cronies, and therefore arrived at Hogwarts with a small gang of friends already made, including Theodore Nott and Vincent Crabbe.
- Draco Malfoy (wizardingworld.com)
Therefore, Draco could be considered a willing follower of the Dark Lord before he became a Death Eater, since he had supported the Dark Lord’s aims for quite some time before officially becoming one of the Death Eaters himself.
