DOSSIER · 13F-HR · Q1 2026

MASON CAPITAL MANAGEMENT LLC

$613M in tracked AUM across 7 positions as of Q1 2026.

MC
CIK 0001218735 · last filed Mar 31, 2026
Total AUM
$613M
as of Q1 2026
Holdings
7
positions
Whale Score
73
73
strong
Activity
no data
changes this Q
AI Analysis · Q1 2026
Comprehensive Analysis of Mason Capital Management LLC 13F Filing
1. Portfolio Overview

Mason Capital Management LLC exhibits a highly concentrated investment strategy with this 13F filing for the period ending September 30, 2025. The firm manages a total portfolio value of approximately $470.38 million, spread across just five holdings. This represents a decrease of 8.0% from the previous quarter's value of $511.53 million. The reduction in portfolio value is likely attributable to market...

2. Key Holdings

The portfolio is dominated by a single massive position in ATS Corporation, which accounts for 86.69% of the total portfolio value ($407.77 million). With 15.56 million shares held, this is clearly the firm's primary bet. Such a heavy concentration in a single name indicates an activist or deep-value inclination where the firm seeks to exert influence or capture a specific upside scenario associated with ATS.

The second largest holding is Kenvue Inc. (KVUE), representing 8.95% of the portfolio ($42.12 million). This provides a degree of stability and exposure to the consumer healthcare sector. The third position, Clear Channel Outdoor Holdings (CCO), comprises 3.30% ($15.53 million) of the assets. Notably, the smallest positions are Mesa Laboratories Inc. (MLAB) at 1.00% and a negligible position in Signature Bank at 0.06%, the latter of which likely represents a residual claim or special situation given its minimal size relative to the total AUM.

Quarter at a glance — Q1 2026

Position-change comparison pending.

No quarter-over-quarter changes available.

Top 10 holdings

By portfolio weight as of Q1 2026.

#HoldingValueSharesWeight
01
ATS
ATS CORPORATION
$463M16.4M75.5%
02
NVRI
ENVIRI CORP
$88M4.5M14.4%
03
BKD
BROOKDALE SENIOR LIVING INC
$31M2.2M5.0%
04
CCO
CLEAR CHANNEL OUTDOOR HLDGS
$23M9.8M3.8%
05
RC
READY CAPITAL CORP
$7M4.3M1.1%
06
JELD
JELD-WEN HOLDING INC
$963K777K0.2%
07
SBNY
SIGNATURE BANK
$260K400K0.0%

Filing history

2026Q1Mar 31
$613M 9.1%
7 positionsView →
2025Q4Dec 31
$562M 19.4%
9 positionsView →
2025Q3Sep 30
$470M 8.0%
5 positionsView →
2025Q2Jun 30
$512M 19.7%
4 positionsLocked
2025Q1Mar 31
$427M 9.2%
5 positionsLocked
2024Q4Dec 31
$471M 6.4%
4 positionsLocked
2024Q3Sep 30
$503M 6.6%
6 positionsLocked
2024Q2Jun 30
$538M 26.1%
6 positionsLocked
2024Q1Mar 31
$728M 18.6%
6 positionsLocked
2023Q4Dec 31
$894M 23.1%
7 positionsLocked
2023Q3Sep 30
$1.2B 13.3%
8 positionsLocked
2023Q2Jun 30
$1.0B 338.3%
10 positionsLocked
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