Johnston started his career as a software engineer doing high-frequency algorithmic trading, and later returned to
South Africa to do venture capital. Later, from 2019 to 2021, Johnston worked as an associate at
McKinsey & Company where he worked with national space agencies. In March 2021, Johnston co-founded
Opontia alongside Manfred Meyer, a digital brand aggregator focused on the Central & Eastern Europe, Middle East, and Africa (CEEMEA) region. The company's business model centered on acquiring profitable digital brands. In June 2021,
Opontia raised $20 million in seed funding from
Global Founders Capital, Presight Capital, Raed Ventures, and Kingsway Capital, along with angel investors including Tushar Ahluwalia, Jonathan Doerr (co-founder of
Jumia), and Hosam Arab. In December 2021,
Opontia raised $42 million in a Series A round, making it one of the largest Series A rounds in the region. The round was a mix of equity and venture debt, with STV leading the equity investment. The company ranked 12th on Forbes Middle East's Top 50 most funded startups in 2021. Opontia was acquired by Perfection in 2023. In early 2024, Johnston co-founded
Starcloud (originally named Lumen Orbit). Starcloud is a US–based company that designs, builds, and deploys
orbital data centers in space.
Starcloud participated in
Y Combinator's Summer 2024 cohort and raised approximately $21 million in seed funding by the end of 2024. Investors include Y Combinator, NFX and
In-Q-Tel, and the company is part of NVIDIA's Inception Program. The satellite successfully trained
NanoGPT, a large language model, marking the first time an
LLM was trained in space. On March 13th, 2026, the
FCC accepted for filing, an application by
Starcloud for a constellation of 88,000 orbital data center satellites. On March 30th, 2026,
Starcloud announced that it had raised a $170M Series A at a $1.1bn valuation led by
Benchmark and
EQT Ventures, making Starcloud the fastest
unicorn in
Y Combinator history just 17 months after completing the program. ==Personal life==