The group said alongside its annual results that money pumped into infrastructure, clean energy, commercial real estate and residential property across British cities by its investment arm and an increase in international assets helped it reach the landmark figure.
Legal & General Investment Management saw assets under management rise 3 per cent to £1.02 trillion.
Boss Nigel Wilson flagged the negative impact political uncertainty was having on asset prices, but nevertheless said L&G performed strongly.
He said: “2018 saw political uncertainty, asset market declines and slowing economic growth, but we are resilient and performed strongly.
“We became the UK’s first £1 trillion investment manager, executed a record £9bn of pension risk transfer deals and invested billions in the UK’s future infrastructure and cities.”
Full-year figures showed that the FTSE 100 company saw operating profit rise 10 per cent to £1.9bn in 2018.
The firm said it was helped by customers dying earlier than expected, or what it called a “heavier than expected mortality experience in 2018”.
L&G released £433m of cash reserves.
Net profit fell 3 per cent to £1.83bn after the prior year was boosted by a one-off US tax benefit.
Legal & General booked record annuity sales of £10bn in its pensions business, while its insurance arm saw gross written premiums rise 3 per cent to £2.62bn.
In the US, protection saw new business annual premiums grow by 12 per cent and international assets jumped 13 per cent to £258bn.
“Our strategy positions us well despite the broader environment, our current trading is strong and we expect this momentum to continue in 2019,” Mr Wilson added.