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Coronal mass ejections (CMEs) can create significant disruption to human activities and systems on Earth, much of which can be mitigated with prior warning of the upstream solar wind conditions. However, it is currently extremely challenging to accurately predict the arrival time and internal structure of a CME from coronagraph images alone. In this study, we take advantage of a rare opportunity to use Solar Orbiter, at 0.5 au upstream of Earth, as an upstream solar wind monitor. In combination with low‐latency images from STEREO‐A, we successfully predicted the arrival time of two CME events before they reached Earth. Measurements at Solar Orbiter were used to constrain an ensemble of simulation runs from the ELEvoHI model, reducing the uncertainty in arrival time from 10.4 to 2.5 hr in the first case study. There was also an excellent agreement in the Bz profile between Solar Orbiter and Wind spacecraft for the second case study, despite being separated by 0.5 au and 10° longitude. The opportunity to use Solar Orbiter as an upstream solar wind monitor will repeat once a year, which should further help assess the efficacy upstream in‐situ measurements in real time space weather forecasting. Plain Language Summary: Coronal mass ejections (CMEs) are large eruptions of plasma from the Sun that can significantly disrupt human technology when directed toward Earth. Much like weather on Earth, the consequences of these "space weather" events can be lessened with warning of their arrival, for example, putting satellites into safe mode. This is usually done by identifying a CME in telescope images, and then predicting if and when it will arrive at Earth. However, the current forecasting models have large uncertainties in arrival time, and struggle to predict the in situ properties of the CME, which can significantly alter the severity of the event. In this paper, by taking advantage of a period in March 2022, we were able to use real time measurements from halfway between the Sun and the Earth taken by the Solar Orbiter spacecraft. This allowed us to predict the arrival time of a CME more than a day before it arrived at Earth, a significant improvement on the current capabilities. We also show that the Solar Orbiter measurements can be used to constrain a CME propagation model, significantly improving the accuracy and precision of the forecasted arrival time. Key Points: Real time data from Solar Orbiter and STEREO‐A were used to predict the arrival time of two coronal mass ejections before arrival at EarthFor one event, in situ measurements at 0.5 au were used to reduce the error in arrival time from 10.4 to 2.5 hr with the ELEvoHI modelThe in situ Bz profile was comparable to the geomagnetic response at Earth, despite being separated by 0.5 au and 10° longitude [ABSTRACT FROM AUTHOR]