Woody Johnson's intervention on Tuesday is the latest in the American attacks on proposals that could see the Chinese firm access "non-core" parts of the new UK network.
He warned against "letting untrustworthy countries" into the infrastructure, and also appeared to back down on an earlier claim that the health service could play a part of a post-Brexit trade deal.
Referencing the "5G question we are all wrestling with right now", the Donald Trump ally said: "This is something which we've discussed a lot and it's very important.
"If you let untrustworthy countries in the heart of our economics and infrastructure, what could they do when inside them? I've always said it's like letting a kleptomaniac move into your house.
"And then you've got to hire three people to follow them around all day and see how that works - it's not a very good situation, maybe that's unfair in some ways but i think it's one way to look at it."
Mr Johnson was subject to fierce rebukes early this month when he said the UK's "entire economy" - including the health service - would be part of a post-Brexit trade deal.
But speaking at a London conference for the Centre of Policy Studies centre-right think tank, he praised the UK's enduring strength.
"I've heard some people say that's naive. They suggest countries like America will bully Britain into a pre-negotiation and a trade negotiation," he said.
"They even claim that we will privatise the NHS. None of that's correct. I promise you that.
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"First, nobody can push Britain around. Britain is a tough country. More than capable of negotiating a deal that protects its own interests."
"Second," he continued, "Britain's success is America's success. The UK is our strongest partner and we want to keep it that way."
He also sought to allay fears of chlorine-washed chicken entering the UK market if a trade deal is brokered, praising the US for thinking "the customer is king".
The practise is banned in the EU, and critics say the process can be used to correct poorer hygiene standards earlier in the production process.
Mr Johnson said: "Our customers are smart, they know what they want and they are capable of making their own decisions. If they don't like something they simply don't buy it.
"British customers are the same, so let them make their own choices."
