The Dane only took over from 2014 team-mate Jenson Button for the afternoon running, but still managed to clock up 52 laps and set the fastest time so far at Jerez, a 1m23.267s.
He played down the significance of that time however, saying that there are still significant areas of improvement for every team and that his focus had therefore been on helping the team hone its future development ideas.
"I'm not looking into the times too much - it's winter testing, so nobody really knows what the teams are doing," Magnussen said.
"I was pushing but it wasn't balls to the wall. We will go quicker.
"I'm pretty happy about what we did, but I'm especially happy about the laps we got in - and the lap Button did too.
"The reliability of the car has been much better than we thought it would be.
"Hopefully that continues - it might not - and we'll continue learning, [because] we need to build up an idea of where the problems will be.
"The important thing is we have a good idea of which direction to go and where to improve."
Jerez F1 test in pictures
Magnussen admitted he had been nervous ahead of his full-time F1 debut, but said previous F1 experience - he tested for McLaren in 2013 and '12 - meant his transition was ultimately seamless.
"I had butterflies in my stomach and I was struggling to sleep, but I'm a rookie you know, I'm excited," he explained.
"It's a massive opportunity for me, and this day was big for me as the first test as a race driver.
"But I don't feel like it is the first day at work; I have been in a Formula 1 car before, I've worked with the team for a few years now and a few months as a race driver.
"I enjoy it a lot, and I feel at home; I don't feel new here."