The Planetary Boundary Layer height (PBLH) is essential for studying PBL and ocean‐ atmosphere interactions. Marine PBL is usually defined to include a mixed layer (ML) and a capping inversion layer. The ML height (MLH) estimated from the measurements of aerosol backscatter by a lidar was usually compared with PBLH determined from radiosondes/dropsondes in the past, as the PBLH is usually similar to MLH in nature. However, PBLH can be much greater than MLH for decoupled PBL. Here we evaluate the retrieved MLH from an airborne lidar (HSRL‐2) by utilizing 506 co‐located dropsondes during the ACTIVATE field campaign over the Northwest Atlantic from 2020 to 2022. First, we define and determine the MLH and PBLH from the temperature and humidity profiles of each dropsonde, and find that the MLH values from HSRL‐2 and dropsondes agree well with each other, with a coefficient of determination of 0.66 and median difference of 18 m. In contrast, the HSRL‐2 MLH data do not correspond to dropsonde‐derived PBLH, with a median difference of 47 m. Therefore, we modify the current operational and automated HSRL‐2 wavelet‐ based algorithm for PBLH retrieval, decreasing the median difference significantly to 8 m. Further data analysis indicates that these conclusions remain the same for cases with higher or lower cloud fractions, and for decoupled PBLs. These results demonstrate the potential of using HSRL‐2 aerosol backscatter data to estimate both marine MLH and PBLH and suggest that lidar‐derived MLH should be compared with radiosonde/ dropsonde‐determined MLH (not PBLH) in general. Plain Language Summary The Planetary Boundary Layer Height (PBLH) is essential for studying the lower atmosphere and its interaction with the surface. Usually, it contains a mixed layer (ML) with vertically well‐mixed (i.e., nearly constant) specific humidity and potential temperature. Over the ocean, the PBL is usually coupled (vertically well‐mixed) and the ML height (MLH) is usually close to PBLH, hence the MLH estimated from the measurements of aerosol backscatter by a lidar is traditionally compared with PBLH determined from radiosondes/dropsondes. However, when the PBL is decoupled (not vertically well mixed), the MLH differs from the PBLH. Here we used dropsondes' thermodynamic profile to evaluate the airborne High‐ Spectral‐Resolution Lidar—Generation 2 (HSRL‐2) estimation of MLH and PBLH in airborne field campaign over the northwestern Atlantic (ACTIVATE) from 2020 to 2022. We show that the HSRL‐2 has excellent MLH estimation compared to the dropsondes. We also improved the HSRL‐2 estimation of PBLH. Further data analysis indicates that these conclusions remain the same for cases with different cloud fractions, and for decoupled PBLs. These results demonstrate the potential of using HSRL‐2 aerosol backscatter data to estimate both marine MLH and PBLH and suggest that lidar‐derived MLH should be compared with radiosonde/ dropsonde‐determined MLH (not PBLH) in general.